The present invention relates to the field of combustion-gas turbines.
Combustion gas turbines are well-known devices for powering electrical generators and other equipment. Combustion gas turbines typically have at least one compressor, one expander and at least one combustor. In the compressor, air is pressurized to mix with a fuel for burning. The air/fuel mixture is then burned in the combustor, and the hot flue gas from the combustor is used to drive the expander.
A range of fuels have been used in combustion gas turbines, including high and low caloric value fuels. High caloric value fuels are defined for purposes of this disclosure as fuels having a heating value of around 20,000-23,000 BTUs per pound. High caloric value fuels include natural gas, which has a caloric value of approximately 23,000 BTUs per pound. Low caloric value fuels are defined for purposes of this disclosure as fuels having a heating value of about 4,000 BTUs per pound or less. Low caloric value fuels include coke oven gas, coal gas, reformed petroleum product gas and blast furnace gas, the latter typically having a caloric value of approximately 1,150 BTUs per pound.
When burned without excess air, high caloric value fuels can burn at about 4,000.degree. F. Temperatures of that order of magnitude, however, are too high for the materials used in today's expanders and would damage the expander. To overcome this problem, combustion gas turbines designed for use with high caloric value fuels typically employ excess compressed air to cool and avoid damage and/or destruction to the expander. In contrast, low caloric value fuels burn at only about 2,000.degree. F. This falls within the temperature constraints of materials used in modern expanders, and excess compressed air is not required.
Under certain circumstances, it may be desirable to use a low caloric value fuel in a combustion gas turbine designed for high caloric value fuels. For example, price or availability differences among different fuels, or stricter atmospheric pollution standards may favor use of a low caloric value fuel. One of the problems caused by using low caloric value fuel in a combustion turbine designed to burn high caloric value fuels is that the design of the combustor requires far more compressed air than is needed to burn a low caloric value fuel. Since using excess air to burn low caloric value fuel would likely extinguish the flame, the excess compressed air must be disposed of to achieve proper functioning.
Several solutions to this problem have been devised. The simplest solution is to bleed the excess compressed air into the atmosphere. This solution is undesirable because the excess air contains considerable energy which would be wasted. Typical excess air may have a pressure of about 200 psig and a temperature of about 660.degree. F. Venting of such high energy air into the atmosphere is extremely inefficient.
Other solutions presently being practiced use the excess air for miscellaneous mechanical purposes besides operation of the turbine. For example, some of the compressed air energy can be recovered in an air expander, which in turn provides power for a generator or other machinery. Alternatively, the residual compressed air can be used in various process applications. Steel plants are in operation which might use the residual compressed air as the "wind" in a blast furnace. Excess compressed air may also be used as feed to an oxygen plant in a gasification unit. However, some loss in efficiency will always be present because the amount, pressure and temperature of the excess air produced is unlikely to match exactly the process requirements for the excess air.
Another solution which has been proposed is to specifically design a compressor for use with a low caloric value fuel in combination with a standard, high caloric expander. This solution involves high design costs and lengthy delays, and results in a machine that would require major modification to burn alternate fuels.
Still another solution is to mix low caloric value fuels with one or more fuels having a higher caloric value. The mixed fuels may have a sufficient caloric value to operate an unmodified turbine without stalling. However, this solution is inherently inefficient because the turbine is not optimized to the air/fuel mixture being used.